1. Field of the Invention
The present invention relates to a variable ratio steering system capable of varying the ratio (steering angle ratio) of the angle through which steerable road wheels are steered to the angle through which a steering wheel is turned by the driver.
2. Description of the Prior Art
One conventional variable ratio steering mechanism for use in a motor vehicle is disclosed in U.S. Pat. No. No. 4,658,927, for example. The disclosed variable ratio steering mechanism includes a planetary gear mechanism disposed in a steering force transmitting path between a steering wheel and a steering gear mechanism. The planetary gear mechanism comprises a sun gear coupled to the steering wheel, a ring gear coupled to the steering gear mechanism, and a carrier coupled to a motor. The motor actuates the carrier depending on the speed of the motor vehicle to vary the steering angle ratio.
In the known variable ratio steering mechanism, the sun gear, the carrier, and the ring gear are rotatable independently of each other. Therefore, it is difficult to put the neutral position of the steering wheel in conformity with the neutral position of the steerable road wheels. If the motor malfunctions, particularly if the motor remains energized out of control, then the carrier is actuated to steer the steerable road wheels even without the steering wheel being turned by the driver.